Sintered tungsten carbide inserts are regularly used in the rolling cutter of rotary drill bits. Although such materials are highly wear resistant, because of the severe conditions in which the bits operate, inserts become dull or blunted with use, resulting in inefficiency and increased energy requirements to accomplish drilling or requiring removal of the bits from service and replacement with new ones.
Generally, the prior art has consistently attempted to overcome the problems associated with wear of drill bit inserts by fabricating the inserts from more abrasion resistant grades of tungsten carbide. The grade of the tungsten carbide is selected depending upon the formation to be cut and conditions encountered in any particular installation. A bit having high abrasion resistance will have a greater wear life but is more brittle and thus more susceptible to fracture. Thus, under severe cutting conditions, a relatively tough grade of carbide may be selected to reduce the tendency of an insert to fracture. However, while the tougher grades of tungsten carbide are less brittle, they are also relatively soft, having less resistance to wear, and therefore, the rotary drill bit will have a short life due to blunting of the insert tip.
Although prior art rotary drill bits have combined two tungsten carbide materials in a single bit to change wear characteristics, the approach taken by prior devices has been to apply a layer of harder grade tungsten carbide on the wear face of the insert with a softer grade therebehind. Examples of bits having this design are disclosed in U.S. Pat. No. 4,194,790 issued to Kenny, et al. and U.S. Pat. No. 4,359,335 issued to Garner. The patent issued to Garner is to an insert designed for the gage row of a rotary drill bit, the harder tungsten carbide being applied to the face defining the gage of the bore hole being drilled. In the Kenny, et al. patent, the harder material is placed on the forward or cutting surface.
It will be noticed that these dual component rock bit inserts of the prior art employ a relatively thin layer of a harder grade of tungsten carbide on a relatively thicker base of a tougher carbide material and the harder grade of tungsten carbide forms the earth engaging face of the insert. The useful life of these hybrid rock bit inserts is limited by the relative thickness of the harder carbide material, with the effectiveness of the inserts being greatly reduced once the harder material has been worn to a blunt surface or completely removed by wear or fracturing during drilling. Further, the hybrid dual component rock bit inserts of the prior art have a low drilling efficiency due to the normal wear of the harder carbide component, which component tends to be blunted or broken during use.
The rock bit inserts of the prior art, whether hybrid dual component inserts, wherein the harder component provides the earth engaging surface, or single component inserts, have a crest-like tip which produces a greater stress on the contacted rock. Although this profile provides for more efficient and effective drilling, the insert tips of the prior art, if formed of a relatively soft tungsten carbide, have the disadvantage of rapidly dulling or blunting during normal use or of fracturing to an inefficient drilling configuration if fabricated from relatively harder carbide materials.